Introduction to SQL
Which of the following query finds the name of the sailors who have reserved at least two boats?

SELECT DISTINCT s.sname FROM sailors s, reserves r1, reserves r2 WHERE s.sid = r1.sid AND r1.sid = r2.sid AND r1.bid <> r2.bid
SELECT DISTINCT s.sname FROM sailors s, reserves r1, reserves r2 WHERE s.sid = r1.sid AND r1.sid = r2.sid AND r1.bid ≠ r2.bid
All of these
SELECT DISTINCT s.sname FROM sailors s, reserves r1, reserves r2 WHERE s.sid = r1.sid AND COUNT(r1.bid) > r2.bid

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Introduction to SQL
If a query involves NOT, AND, OR with no parenthesis

The order of occurrence determines the order of evaluation.
NOT will be evaluated first; AND will be evaluated second; OR will be evaluated last.
NOT will be evaluated first; OR will be evaluated second; AND will be evaluated last.
AND will be evaluated first; OR will be evaluated second; NOT will be evaluated last.

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Introduction to SQL
NULL is

the same as 0 for integer
the same as blank for character
not a value
the same as 0 for integer and blank for character

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